In the foregoing medical systems for guiding, e.g., a biopsy needle to a target in body tissue, the part of the body in which the tissue is located is usually imaged by an ultrasound transducer which scans the body along scan planes. The location and orientation of a rigid straight needle, or other insertion device, is determined by a position sensor secured at a predetermined location on the needle. The absolute location and orientation of the plane displayed by the imaging system must also be determined, this being done by a position sensor secured at a given convenient location on the ultrasound scanning transducer. The system enables the measurement of the relative location and orientation of the needle with respect to the target tissue also to be calculated. Once these values are determined, it is possible to compute the expected path of the needle towards the target and to display it on the image in order to enable the physician to navigate the needle precisely towards the target.
An example of such an imaging system is described in our patent application No. PCT/IL96/00050 published Feb. 6, 1997, which is hereby incorporated by reference.
In such a system, the position sensor, secured to a predetermined point on the needle, measures the precise location and orientation of the needle upper tip, but the position sensor, being attached to the ultrasound transducer at a convenient, arbitrary location thereon, does not have a well determined spatial position and orientation to the scan plane of the transducer so as to precisely relate the transducer position sensor to the transducer scan plane. Yet, since the navigation of the needle to the target uses the ultrasound image as a background for the display of the future path of the needle, it is imperative to calculate the precise location and orientation of the scan plane with respect to the position sensor on the ultrasound transducer.
A method described in relation to calibrating a magnetic position sensor being affixed to an ultrasound transducer is described in, Detmer et al., in "3D. Ultrasonic image Feature Localization Based on Magnetic Scanhead Tracking": In Vitro Calibration and Validation. This method has the drawback that the ultrasound transducer must be maneuvered at a relatively large number of positions, and complicated mathematical algorithms are employed in order to resolve ultrasound beam plane orientation with respect to the target. The methods presented herein alleviate these aforementioned drawbacks, by utilizing ultrasound target(s) that are known and fixed spatially with respect to a position measuring component.